Placing particulate material at a location below ground is a very significant part of a hydraulic fracturing operation. Hydraulic fracturing is a well established technique for reservoir stimulation. Fluid is pumped under pressure into a subterranean formation, forcing portions of the formation apart and creating a thin cavity between them. When pumping is discontinued the natural pressure in the subterranean formation tends to force the fracture to close. To prevent the fracture from closing completely it is normal to mix a solid particulate material (termed a proppant) with the fracturing fluid at the surface and use the fluid to carry the proppant into the fracture. When the fracture is allowed to close, it closes onto the proppant and a flow path to the wellbore between the proppant particles remains open. The proppant is then under considerable pressure from the formation rock pressing on it.
It is normal practice to employ solid proppant of controlled particle size distribution in order that the proppant pack has adequate fluid conductivity, i.e. is adequately porous, and to mitigate the flowback of fine particles. Post-fracture proppant flowback to the wellbore is generally regarded as a problem and an occurrence to be avoided. Although many materials have been used as proppants, for the fracturing of oil reservoirs it is commonplace to use so-called 20/40 sand which has a particle size distribution such that 90% by weight passes a 20 US mesh (840 micron) sieve but is retained by a 40 mesh (400 micron) sieve. Finer materials have been used and American Petroleum Institute Recommended Practices (API RP) standards 56 and/or 60 recognize proppant sizes down to a size range of 70/140 US mesh (sieve openings of 210 and 105 micron). Materials which are smaller than 70/140 US mesh have been regarded as too small to use as proppants.
When the proppant is mixed with the fracturing fluid at the surface and pumped into the wellbore it is subjected to very high shear. The proppant-laden fluid then flows down the wellbore under conditions of lower shear. Subsequently it turns and flows out of the wellbore and into the fracture in the formation. Entry to the fracture may be associated with an increase in shear, in particular if the wellbore is cased and the fluid passes through perforations in the wellbore casing to enter the fracture. Once the fluid enters the fracture, and as the fracture propagates and extends into the reservoir, the fluid is subjected to much less shear. Suspended solid begins to settle out. Subsequently pumping is discontinued, allowing the fracture to close onto the proppant packed in the fracture.
In order that the fluid can convey particulate material in suspension, and place it across the fracture face, it is conventional to include a viscosity-enhancing thickening agent in the fluid. Typically the fluid is then formulated so as to achieve a viscosity of at least 100 centipoise at 100 sec−1 at the temperature of the reservoir. Guar is widely used for this purpose. Guar derivatives and viscoelastic surfactants may also be used. However, for some fracturing operations, especially where the rock has low permeability so that leak off into the rock is not a significant issue, it is preferred to pump a fluid, often called “slickwater”, which is water or salt solution containing a small percentage of friction reducing polymer. The fluid then has low viscosity. This considerably reduces the energy required in pumping but keeping particulate material in suspension becomes much more difficult and a higher pump flow rate is commonly used.
As recognized in Society of Petroleum Engineers Papers SPE98005, SPE102956 and SPE1125068 conventional proppant particles suspended in slickwater pumped into a large fracture will settle out more quickly than is desired and form a so-called “bank” or “dune” close to the wellbore. Because of this premature settling, proppant may not be carried along the fracture to prop the full length of the fracture and proppant may not be placed over the full vertical height of the fracture. When pumping is stopped and the fracture is allowed to close, parts of the fracture further from the wellbore may not contain enough proppant to keep them sufficiently open to achieve the flow which would be desirable. As a result the propped and effective fracture size may be less than the size created during fracturing.
One approach to improving the transport of particulate proppant has been to use a material of lower specific gravity in place of the conventional material which is sand or other relatively heavy mineral (sand has a specific gravity of approximately 2.65). SPE84308 describes a lightweight proppant having a specific gravity of only 1.75 which is a porous ceramic material coated with resin so that pores of the ceramic material remain air-filled. This paper also describes an even lighter proppant of specific gravity 1.25 which is based on ground walnut hulls. This is stated to be a “resin impregnated and coated chemically modified walnut hull”.
These lightweight proppants are more easily suspended and transported by slickwater and their use is further discussed in SPE90838 and SPE98005, the latter paper demonstrating that settling out is reduced compared to sand, although not entirely avoided. There have been a number of other disclosures of proppants lighter than sand. Examples are found in U.S. Pat. Nos. 4,493,875 and 7,491,444 and in US patent applications 2005/096,207, 2006/016,598 and 2008/277,115.
A recognized issue with lightweight proppants is that they may not be as strong as sand and are at risk of becoming partially crushed when a hydraulic fracture is allowed to close on the proppant placed within it. An approach to the suspension of particulate proppant which seeks to avoid this issue is disclosed in US2007/015,669, also in WO2009/009,886 and in “Lightening the Load” New Technology Magazine, January/February 2010 pages 43 and 44. According to the teachings of these documents, a conventional proppant such as sand is treated to render its surface hydrophobic and is added to the slurry of proppant and water. Bubbles adsorb to the hydrophobic solid particles so that the adsorbed gas gives the particles a lower effective density. The literature describing this approach advocates it on grounds that the conventional sand is both cheaper and stronger than lightweight proppant.